Direct thermal imaging is widely used for printing variable information; for example, this imaging method is commonly used to print facsimiles, receipts, shipping address labels, barcodes and prescription labels. Direct thermal imaging or printing is accomplished by directing heat to specific regions of thermosensitive coated substrate resulting in a change in the color of the region which was heated. Imagewise heating of the thermosensitive substrate is accomplished using a thermal printer such as, for example, the printers provided by Zebra Corporation of 475 Half Day Road, Suite 500, Lincolnshire, Ill. 60069. Such printers contain thermal printheads comprised of linear arrays of individually addressable heating elements, typically containing 60 to 236 such heating elements per linear centimeter of printhead. The thermal printhead is placed in intimate contact with the thermosensitive substrate. As the substrate is caused to move beneath the printhead, the individual heating elements are caused to heat in an imagewise pattern, imaging one complete line across the thermosensitive substrate at a time. Typical printing speeds range from 2.5 centimeters per second cm/s to 30 centimeters per second.
Direct thermal imaging has been widely accepted as a fast and efficient digital printing method. However, this printing technology has a significant weakness, i.e., the stability of the printed image to fading from exposure to sunlight. Thus, e.g., U.S. Pat. No. 6,034,704 of Stewart discloses that thermally activated substrates produce images which can be expected to fade. Labels, facsimiles and receipts printed on direct thermal sensitive substrates will fade quickly if they are not stored in a dark environment. Many labeling applications require the printing of variable information onto substrates for outdoors usage and consequently require good resistance to fading induced by exposure to sunlight.
It is an object of this invention to provide a thermographic substrate assembly that affords good resistance to fading induced by exposure to sunlight.